Re: Howto avoid full re-sync

[John Robinson <john.robinson@anonymous.org.uk>]

On 12/09/2012 13:52, Adam Goryachev wrote:
> On 11/09/12 00:12, Ralf Müller wrote:
>> Besides all the stuff about fix your server, a raid is not a backup and you risk your data - simply add a write intent bitmap:
>>
>> mdadm /dev/md2 --grow --bitmap=internal
> I've added this across a number of my systems now, and it seems to work
> really well (Thank You), especially one system which has a RAID1 with an
> external USB drive + internal drive which normally take over 2 days for
> a full resync.

It may also be worth reading up on --write-mostly for your external USB 
drive, particularly if your workload tends to be single long streaming 
reads rather than lots of small parallel ones.

> Can you suggest if there is any dis-advantage to using the bitmap (maybe
> write performance will suffer), or disk space is reduced, or ....
>
> While I can see the benefits, I'm just wondering if it might be too good
> to be true, or what I am missing....
>
> Thanks again for your assistance.

Yes, write performance suffers, especially if you have a small bitmap 
chunk size, and the default is as small as possible for the array. See 
`mdadm -X /dev/sdX` on one of the components of your array for what the 
default was calculated to be for your array, and read the --bitmap-chunk 
section of `man mdadm` for a description of bitmap chunk size 
considerations. I found a bitmap chunk of 128MB (131072KB) kept write 
perfomance very near no-bitmap speeds (both MB/s and IOPS) but kept 
resync times fast (a few seconds).

I'm not sure, but you may have to remove the bitmap (--grow 
--bitmap=none) before re-adding one with a different chunk size (e.g. 
--grow --bitmap=internal --bitmap-chunk=131072).

Cheers,

John.

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